weak: don't panic when calling Value on a zero Pointer

Currently weak.Pointer.Value will panic if the weak.Pointer is
uninitialized (zero value) which goes against it's documentation. Fix
this and add a test. While we're here, also add a test to ensure
weak.Make[T](nil) is equivalent to the zero value of weak.Pointer[T].

Fixes #71153.

Change-Id: I4d9196026360bc42a5bfcb33ce449131ec251dba
Reviewed-on: https://go-review.googlesource.com/c/go/+/641095
Reviewed-by: David Finkel <david.finkel@gmail.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

src/weak/pointer.go

@@ -78,6 +78,9 @@ func Make[T any](ptr *T) Pointer[T] {
 // If a weak pointer points to an object with a finalizer, then Value will
 // return nil as soon as the object's finalizer is queued for execution.
 func (p Pointer[T]) Value() *T {
+	if p.u == nil {
+		return nil
+	}
 	return (*T)(runtime_makeStrongFromWeak(p.u))
 }
 

src/weak/pointer_test.go

@@ -21,6 +21,15 @@ type T struct {
 }
 
 func TestPointer(t *testing.T) {
+	var zero weak.Pointer[T]
+	if zero.Value() != nil {
+		t.Error("Value of zero value of weak.Pointer is not nil")
+	}
+	zeroNil := weak.Make[T](nil)
+	if zeroNil.Value() != nil {
+		t.Error("Value of weak.Make[T](nil) is not nil")
+	}
+
 	bt := new(T)
 	wt := weak.Make(bt)
 	if st := wt.Value(); st != bt {
@@ -41,6 +50,12 @@ func TestPointer(t *testing.T) {
 }
 
 func TestPointerEquality(t *testing.T) {
+	var zero weak.Pointer[T]
+	zeroNil := weak.Make[T](nil)
+	if zero != zeroNil {
+		t.Error("weak.Make[T](nil) != zero value of weak.Pointer[T]")
+	}
+
 	bt := make([]*T, 10)
 	wt := make([]weak.Pointer[T], 10)
 	wo := make([]weak.Pointer[int], 10)